This invention is directed to a moisture detection apparatus for sheets or webs and in particular to a scanning microwave moisture detection apparatus for providing a moisture profile of the sheet or web.
In order to control paper making machines, it is desirable to determine the moisture content of the paper at various stages of the process. To achieve this, either fixed or scanning microwave moisture detectors are used to detect the moisture content of the sheet or web which is moving at a speed from a hundred feet per minute to several thousand feet per minute. One type of sensor which may be used for this purpose includes a fringing field sensing head which is in contact with one side of the sheet. The fringing field interacts with the sheet and microwave power is absorbed. Thus the attenuation of the power in the sensing head is a function of the moisture in the sheet. Another type of sensor includes a transmitter horn on one side of the sheet and a receiver horn on the other side. This is a non-contacting sensor and some of the microwave power transmitted through the sheet will be absorbed by the moisture in the sheet. Thus the power received by the receiver horn is a function of the moisture content of the sheet.
In many instances it is highly desirable to scan the sheet or web along its width and obtain a moisture profile of the sheet. The prior art teaches two types of scanning systems, one being mechanical and the other using electronic switching. In the mechanical system, which is the only one that is in common use, the sensor is physically moved across the width of the sheet. This need for moving parts results in a rather combersome apparatus and provides a profile which varies with the direction of travel of the sheet or web. In the electronic system, as described in U.S. Pat. No. 3,534,260, Oct. 13, 1970, C. W. E. Walker, power is switched from one sensor to the next, requiring an elaborate control system. In U.S. Pat. No. 3,470,343, Sept. 30, 1969, J. Bilbrough, both mechanical scanning and electronic sampling of the output from an array of separate moisture gauges is described.